This invention relates generally to heat pump systems and more particularly to an apparatus and method for raising both the heating mode condenser air flow temperature and at the same time decreasing the heating mode condenser duct air flow rate, for a given sensed evaporator ambient air temperature.
Heat pumps are refrigeration systems used in both heating and cooling. Heat pump systems use a refrigerant to carry thermal energy between a relatively hotter side of a circulation loop, where compression of the refrigerant by a compressor raises the temperature of the refrigerant, to a relatively cooler side of the loop at which the refrigerant is allowed to expand, causing a temperature drop. Thermal energy is added to the refrigerant on one side of the loop and extracted from the refrigerant on the other side, due to the temperature differences between the refrigerant and the indoor and outdoor air, respectively, to make use of the outdoor air as a thermal energy source.
Heat pumps are bi-directional, in that suitable valve and control arrangements selectively direct the refrigerant through indoor and outdoor heat exchangers so that the indoor heat exchanger is on the hot side of the refrigerant circulation loop for heating and on the cool side of the refrigerant circulation loop for cooling. A circulation fan passes indoor air over the indoor heat exchanger and through ducts leading to the indoor space. Return ducts commonly extract air from the indoor space and bring the air back to the indoor heat exchanger. A fan likewise passes ambient air over the outdoor heat exchanger, and releases heat into the open air, or extracts available heat therefrom.
These types of heat pump systems can operate only if there is an adequate temperature difference between the refrigerant and the air at the respective heat exchanger so as to maintain a transfer of thermal energy. As the heating mode evaporator ambient air (or outdoor air) temperature decreases, the refrigerant temperature entering the condenser consequently decreases, and the air temperature heated by the condenser and exiting the condenser consequently decreases. At outside air temperatures as high as 50.degree. F., it is typical that the condenser exiting air temperature has already decreased to below 98.degree. F., and will decrease further as the outdoor air temperature declines. Persons exposed to an exiting air flow draft below 98.degree. F. experience a feeling of discomfort that is heightened further when the exiting air temperature drops. The phenomenon of this uncomfortable feeling is commonly referred to as "cold blow". During a cold blow condition, the more rapid the flow rate of the exiting air, the greater the feeling of cold blow discomfort.
It is conventional practice to design a heat pump system primarily for use as a cooling mode apparatus, and consequently optimize heat pump system characteristics for their cooling mode operation characteristics and not their heating mode operation. Specifically, prior art heat pump system indoor heat exchanger fan speeds are optimized for their performance as cooling mode evaporator fans, and the heating performance of the cooling mode evaporator fan speeds is conventionally considered acceptable for the heating mode on the basis of system design economy and capacity performance.
The optimization of heating mode condenser fan speeds for their use as cooling mode evaporator fans results in fans that are generally of a fixed speed that create a greater air flow than is necessary for the heating mode operation. Prior art heat pump system control is accomplished by using a thermostat that cycles the entire system (compressor and fans) on and off in response to a demand for heating, thereby maintaining the temperature inside an enclosure at a desired level. In particular, during operation at relatively cool outdoor temperatures, these fan speeds result in a higher speed enclosure air circulation, and an air cooler than would be obtained with lower condenser fan speed operation, alternatively exacerbating the affect of cold blow, or creating a cold blow situation. The heating capacity supplied by the heat pump to heat the space is sufficient, but it is delivered at a relatively low temperature and at an air velocity which feels drafty. The problem worsens as the outdoor temperature falls and supplementary heat is not required (because the system is above the thermal balance point) to meet capacity needs. The heat pump system capacity decreases and, with constant airflow, the supply air temperature is correspondingly lower, increasing the cold blow affect of the delivered air.
It is also conventional practice to design a heat pump system indoor heat exchanger fan speed to provide at least the desired air flow for a range of indoor heat exchanger duct air drag characteristics, so that at duct air drags less than the maximum designed for duct air drag, the furnished air flow is generally greater than necessary to provide the desired heat exchange.
Accordingly, there is a long-felt need when the outdoor air temperature is low enough to cause a cold blow condition, to raise the temperature of the condenser exiting air, and/or to lower the flow rate of the condenser exiting air.
The conventional means for relieving a cold blow situation is to include a supplementary heater that generates electrical resistive heat disposed in the exiting air path of the heat pump system. U.S. Pat. No. 4,141,408 discloses such a conventional means for increasing the temperature of the exiting air. This particular patent proposes to use sensors positioned on an indoor coil to measure the temperature of the air leaving the coil. The heating elements are turned on and off in response to the temperatures sensed by the sensors. The inclusion of supplementary heaters to a heat pump system add otherwise unnecessary expense and complication to the heat pump system. Furthermore, during the period of supplementary heat dissipation during a cold blow situation, the supplementary heaters significantly increase the expense and consumption of energy to the operational costs of the heat pump system.
It is therefore an object of the present invention to overcome the problems of the prior art described above by providing a system which controls of the indoor air mover in a manner to provide the optimum comfort performance that can be achieved from a heat pump while maintaining reliable compressor operation.
A further object of this invention is to eliminate the feeling of cold blow experienced in the use of heat pumps for residential heating at mid to low outdoor temperatures where the heat pump is satisfying the structure load required without supplementary electric heater use.
Another object of the invention to provide a control method for a heat pump system which will adjust the indoor airflow during periods where auxiliary heat is not required to meet the space load, but where the heat pump capacity is too low to deliver air at a comfortable temperature.
It is a further object of this invention to provide such a control by the use of an outdoor temperature sensor input for direct measurement of the outdoor temperature, and a control algorithm using the measured temperature information to modify the operation of the indoor blower to improve the temperature and velocity of the conditioned air delivered to the space resulting in improved occupant comfort.
Another object of this invention to provide a heat pump system which will respond to thermostat signals in a manner which will result in the improved occupant comfort.
Briefly stated, the objectives of the present invention have been attained by a heat pump operating in the heating mode that controls the condenser air flow rate and the condenser exiting air temperature, depending in a first embodiment upon the evaporator ambient temperature, and in a second embodiment, upon the evaporator air temperature and alternatively the condenser air flow rate or the condenser exiting air temperature. The apparatus and method operate by sensing the evaporator ambient temperature with a sensor positioned proximate to the evaporator, and when that ambient temperature is below a threshold value thus indicating a cold blow situation, determining by circuit means a modified condenser air flow rate to achieve both a slower air flow, and a higher air temperature, terminating the cold blow condition. The apparatus and method alternatively command the blower to achieve a determined condenser air flow, or a determined speed depending upon motor type, that results in a targeted condenser air flow or a targeted condenser exiting air temperature.
According to an apparatus embodiment of the present invention, a heat pump operating in the heating mode having an indoor air exchanger, the condenser, includes a means for moving the supply air over the condenser (generally at least one blower), and a thermostat means that has both a sensor means for determining the outdoor air temperature (more broadly the evaporator ambient temperature) and a means for regulating the flow rate (volume of air per unit of time) of the supply air over the condenser in response to the outdoor air temperature, so that a non-cold blow situation is ensured.
A method embodiment of the present invention includes a sensing step of sensing the evaporator ambient temperature, a determining step of determining at a given evaporator ambient temperature a blower supply air flow characteristic that is consistent with a non-cold blow supply air temperature or a reduced supply air flow rate flow rate that alleviates a cold blow condition, and a transmitting step of transmitting the determined blower supply air flow characteristic to the blower. The term "blower supply air flow characteristic" refers to the characteristic that is used to control the blower motor of the variable speed blower, including alternatively a motor speed command consistent with motor type, or a targeted supply air flow rateb or blower motors that adjust their speed according to a targeted flow rate.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.